FIG. 43 is a perspective view of a thin film solar cell including a thin silicon film on a substrate. In FIG. 43, a substrate 200 supports an active layer 201 including a p-n junction for power generation. An anti-reflection film 202 is disposed on the active layer 201 and an upper electrode is disposed on the anti-reflection film 202. The upper electrode comprises a grid electrode 203a for collecting photoelectric current generated in the active layer 201 and a bus electrode 203b concentrating the current from the grid electrode 203a. A lower electrode 204 is disposed on the rear surface of the substrate 200.
In this thin film solar cell, the active layer 201 is only several tens of microns thick and, therefore, cannot support itself so that some supporting substrate is required. The substrate must be able to support the thin film and itself. Since the silicon active layer is grown on the substrate 200 by thermal chemical vapor deposition (CVD) or the like, the substrate must be refractory so that it can stand a processing temperature of approximately 1,000.degree. C. Since the substrate also is an electrode, it should be electrically conductive. If the substrate is not electrically conductive, it is necessary to contact the lower electrode by disposing an electrically conductive film on the substrate or by producing an integrated solar cell, resulting in a complicated structure. Since the substrate itself does not contribute to power generation but only supports the active layer, the substrate should be inexpensive.
Substrates comprising steel, graphite, metallurgical grade silicon, or the like meet the above-described requirements to some extent, as discussed in 14th IEEE Photovoltaic Specialists Conference, (PVSC14), page 281 (1980), by L. L. Kazmerski. A steel substrate or metallurgical grade silicon substrate includes a large quantity of impurities, such as Fe, that adversely affect the solar cell characteristics even when present in a very small amount. Therefore, these materials are not suitable for a thin film solar cell substrate. Kazmerski describes the graphite substrate as the most suitable of the substrate that satisfy the foregoing requirements.
FIG. 44 is a cross-sectional view showing a prior art thin film solar cell having a graphite substrate, disclosed in the Journal of the Electrochemical Society, 123, page 106 (1976) by Chu et al. This thin film solar cell has the same structure as that shown in FIG. 43 and FIG. 44 corresponds to part of a cross-section taken along line 44--44 of FIG. 43. In FIG. 44, a conductive substrate 205 comprises sintered graphite supporting a polycrystalline silicon thin film active layer 206. An anti-reflection film 207 is disposed on the active layer 206. An upper electrode 208 is disposed on the anti-reflection film 207 and a lower electrode 209 is disposed on the rear surface of the substrate 205.
The graphite substrate is formed from graphite powder obtained from anthracite, coke, or the like placed in a mold and sintered at a temperature of about 3,000.degree. C. Then, the graphite is removed from the mold and polished to make its surface flat, completing the graphite substrate.
The thus-formed graphite substrate is placed in a CVD apparatus and silane (SiH.sub.4) or silane trichloride (SiHCl.sub.3) is introduced into the apparatus and reacted at a temperature of about 1,000.degree. C. whereby a polycrystalline silicon film (active layer 206) having a thickness of several tens of microns is groom on the graphite substrate 205. The grown thin silicon film has small diameter crystal grains so that, in some cases, the polycrystalline silicon is melted and recrystallized by laser radiation or lamp heating to increase the diameters of the crystal grains. After forming the film, a p-n junction is produced in the active layer 206 by impurity diffusion or ion implantation. The p-n junction may be produced by changing the dopant gas while the active layer is formed using dopant source gases in the CVD process or by depositing, in a plasma CVD process, a microcrystalline film having a conductivity type opposite that of the active layer.
After forming the p-n junction, the anti-reflection film 207 is formed by sputtering or the like. As the anti-reflection film 207, a transparent conductive film also serving as an electrode, such as an ITO (In.sub.2 O.sub.3 :SnO.sub.2) film, an SnO.sub.2 film, or a ZnO film, is used when the conductivity of the silicon film in the transverse direction is low. When the conductivity of the silicon film in the transverse direction is high and the transparent electrode is not required, an insulating film, such as an Si.sub.3 N.sub.4 film, is used as the anti-reflection film. Thereafter, an upper electrode 208 is formed on the anti-reflection film 207. The upper electrode usually comprises silver and it is formed by screen printing or vapor deposition.
The thin film solar cell having the graphite substrate has the following drawbacks.
The graphite substrate is formed in a complicated process requiring manual labor. Graphite powder is placed in a mold and sintered at a temperature of 2,000.degree.-3,000.degree. C. Then, the graphite is removed from the mold and polished to make the surface flat. Therefore, although the graphite powder is inexpensive, the production cost is high because a high temperature process is required. In addition, since the graphite must be placed in the mold and then removed from the mold, continuous production is difficult.
Even if the surface of the graphite substrate is polished, an unevenness of several microns remains, causing electrical leakage, especially when the active layer is thin. FIG. 45 is an enlarged view of a surface portion of the conventional graphite substrate (in FIG. 44, the surface of the substrate 205 in contact with the active layer 206). Reference numeral 201 designates graphite powder. As shown in FIG. 45, the conventional graphite substrate has an unevenness of several microns.
Since the conventional graphite substrate has a low reflectivity, light passing through the active layer is hardly reflected by the surface of the substrate. As a result, it is not possible to effectively utilize the incident solar light.
Since the conventional graphite substrate is porous, it absorbs water from the rear surface when used for a long time, resulting in a deterioration in the active layer.
Since the graphite substrate is not pliable, continuous production utilizing the roll-to-roll method shown in FIG. 33(a) is impossible. In the conventional method, graphite substrates are placed on a susceptor one-by-one and the thin film solar cells are produced one-by-one, resulting in difficulty in continuous mass production.
On the other hand, when the active layer is an amorphous film that can be formed at a temperature lower than 300.degree. C., continuous production can be carried out utilizing a sheet of heat-resistant plastic. However, since the heat resistance temperature of the substrate is approximately 300.degree. C., the conditions for film formation are restricted. In addition, since the substrate has poor moisture resistance, it easily absorbs water.
FIG. 46 shows a method for producing the thin film solar cell disclosed in Japanese Published Patent Application 53-44192. In FIG. 46, a graphite sheet substrate 211 is moved by rollers 212 and 213 through an apparatus including an active layer formation chamber 214, a grain diameter enlargement chamber 215, and a p-n junction formation chamber 216. Source gases for forming the active layer are introduced into the active layer formation chamber 214 through a gas inlet 217. Power sources 219 generating current conducted through terminals 200 are connected to the graphite sheet substrate 211 for heating the substrate. A lamp 221 heats and a cutter 222 cuts the substrate.
The graphite sheet substrate 211 is fed to the active layer formation chamber 214 by the roller 212. A source gas, such as silane or dichlorosilane, for forming the silicon thin film active layer is introduced into the active layer formation chamber 214 through the gas inlet 217. Since the graphite sheet substrate 211 is in contact with the terminals 220, current supplied from the power source 219 flows in and heats the substrate. When the substrate is heated, the source gas reacts and a polycrystalline silicon active layer is deposited on the substrate 211. The reacted gas and unreacted gas are discharged through the gas outlet 218.
The graphite sheet substrate on which the polycrystalline silicon thin film is formed is transferred to the grain diameter enlargement chamber 215 by the roller. In the grain diameter enlargement chamber 215, the substrate is heated by current supplied from the power source 219 and the polycrystalline silicon thin film is heated by the lamp 221 whereby the polycrystalline silicon is melted and recrystallized to increase the grain diameters of the silicon.
In the p-n junction formation chamber 216, a p-n junction is formed in the silicon thin film by impurity diffusion or the like. Thereafter, the substrate 211 is cut in a prescribed length by the cutter 222.
In this prior art structure, a graphite sheet satisfies the above-described requirements, specifically, support of a thin film, refractory behavior, electrical conductivity, and low cost. Sheet graphite has conventionally been used as a refractory gasket or packing, a heat insulating material of a nuclear reactor, or the like.
FIG. 47 shows the crystal structure of sheet graphite. Sheet graphite is made from naturally-occurring flaky graphite and has a crystal structure in which layers 223, having carbon atoms arranged in a plane with six carbon atoms (circles) connected with each other, are laminated. Although the bonds between the carbon atoms in the same plane are strong, bonds between the carbon atoms in different layers 223 are weak because they are van der Waals bonds, so that the graphite easily cleaves into layers. Although the crystal structure of the flaky graphite is similar to that of conventional graphite powder when seen microscopically, the crystal size of the flaky graphite is larger than that of graphite powder and there are cases where crystals are as large as several millimeters.
FIG. 48 shows a method for producing the sheet graphite. Flaky graphite is given an acid treatment in a solution comprising NH.sub.4 OH and H.sub.2 SO.sub.4 and then the acid is evaporated at 300.degree. C. whereby the flaky graphite foams and becomes like cotton, increasing its volume. Foamed graphite 224 is pressed by rollers 225 at room temperature, resulting in a graphite sheet 226. The graphite sheet is formed by applying pressure without high temperature treatment because each of the crystals is relatively large.
Since the graphite sheet is formed by rolling, the crystal structure and the layer structure are laminated in a thickness direction of the substrate. Such an anisotropic structure may be formed by applying pressure from one direction. Since the internal structure of the graphite sheet is anisotropic, its thermal and electrical conductivity are anisotropic in the surface direction and the thickness direction. In addition, the graphite sheet formed at room temperature is flexible and its surface is always smooth.
When the graphite sheet is formed by rolling, the surface configuration of the substrate depends on the surface configuration of the rollers. Therefore, to produce a smooth surface of the substrate, the rollers have smooth surfaces.
FIG. 49 is an enlarged view of a surface portion of the graphite sheet. As shown in FIG. 49, the graphite sheet has a structure in which layers 227, each having one or a plurality of the crystal structures shown in FIG. 47, are laminated. Accordingly, although the flexibility of the substrate causes a waviness of several millimeters to several centimeters, there is little unevenness of the surface so long as it is not damaged physically and the surface is fundamentally very smooth.
Furthermore, since the layers 227 run parallel to the surface of the graphite substrate, the side surfaces of the substrate easily absorb water and air, but water and air hardly percolate in a direction perpendicular to the surface of the substrate. Therefore, even when water is applied to the rear surface of the substrate, water does not affect the active layer.
Since the layers 227 run parallel to the surface of the graphite substrate, the reflectivity of the graphite substrate is higher than that of an ordinary isotropic graphite plate. The reflectivity in the direction perpendicular to the planar crystal structure of the graphite is higher than the reflectivity in the parallel direction.
In the production method illustrated in FIG. 46, a graphite sheet having the above-described characteristics is used as a substrate whereby thin film solar cells with high efficiency and high reliability are produced continuously.
However, the production method shown in FIG. 46 has the following drawbacks. The active layer is formed by lamp heating or electric heating. With lamp heating, since the entire reaction chamber is heated, polycrystalline silicon is deposited on the internal wall of the chamber so that chamber maintenance takes a long time. In addition, since the graphite sheet has a lustrous surface, its reflectivity is large compared to an ordinary sintered carbon plate. Therefore, the heating light is also reflected from the surface so that the heating is not carried out effectively. With electrical heating, the terminals of the power source may not be well connected to the graphite sheet or the temperature of the substrate may be uneven because the current flows unevenly. In addition, since the graphite sheet is very soft, the graphite sheet may be deformed or exfoliate at the connecting portions 228 of the graphite sheet 221 and the terminals 220, as shown in FIG. 50.
Since the graphite sheet substrate is soft and flexible, when a thin film of polycrystalline silicon is formed on the substrate by CVD, the substrate curves, as shown in FIG. 53, due to the difference of thermal expansion coefficients between the silicon 237 and the graphite 236, adversely affecting the following process. When the grain diameter enlargement step is carried out, heat is not evenly applied to the crystalline surface so that the grain diameters are enlarged in only part of the substrate. As a result, the enlargement of grain diameters over the entire substrate is only possible when a very small substrate, about several centimeters square, is used. This problem occurs not only when solar cells are continuously produced using the beltlike graphite sheet, as shown in FIG. 46, but also when solar cells are produced one-by-one using graphite sheets.
In a conventional process for forming a thin film, not limited to the method of FIG. 46, source gases introduced into a reaction chamber are exhausted through an outlet before they are completely reacted which means that the source gases are not effectively used. This loss mitigates an important advantage of a thin film solar cell, that is, a reduction in production cost by decreasing the quantity of semiconductor gases used in the production process.
Because the thin film formation chamber 214 into which the source gases are introduced is large and much maintenance of the chamber 214 is required, production costs are high.
When the grain diameters of the thin film polycrystalline silicon on the graphite sheet substrate are increased by zone melting, in FIG. 46, the polycrystalline silicon condenses due to the curvature or the like of the substrate. Further, the grain diameter enlargement chamber 215 is contaminated with silicon evaporated from the surface of the polycrystalline silicon layer.
A thin film solar cell utilizing a metallurgical grade silicon (hereinafter referred to as MG-Si) substrate 350 is shown in FIG. 54. In FIG. 54, a silicon thin film 351 is disposed on the MG-Si substrate 350. An emitter layer 352 is formed in the surface region of the thin film 351 by impurity diffusion or the like. An upper grid electrode 353 is disposed on the emitter layer 352. The MG-Si substrate is less expensive than crystalline silicon having high purity and never curves when a thin film polycrystalline silicon layer is formed on it. However, the MG-Si substrate is heated to about 1,000.degree. C. when the polycrystalline silicon layer is grown and up to the melting point of silicon (1,414.degree. C.) when the grain diameter is enlarged. Therefore, as shown in FIG. 55, impurities 354, such as Fe, Al, or Ca, in an amount of approximately 2% in the MG-Si substrate 350 diffuse from the surface whereby the active layer is degraded.
FIG. 51 is a cross-sectional view schematically showing a prior art solar cell utilizing a crystalline silicon thin film on a substrate mainly comprising carbon. In FIG. 51, a substrate 230 whose main component is carbon has a silicon layer 231 disposed on it. An emitter layer 232 is disposed on the silicon layer 231 and a metal electrode 233 is disposed on the emitter layer 232.
Since the substrate 230 mainly comprises carbon, molded carbon, or graphite, a graphite sheet or the like may be used as the substrate 230. The substrate 230, mainly comprising carbon, has the following advantages. Since carbon withstands a temperature of about 3,000.degree. C. in a reactive atmosphere, it can withstand the temperature of 1,000.degree.-1,500.degree. C., required for forming a crystalline silicon thin film. Because carbon has good electrical conductivity, it can also serve as a rear electrode of the solar cell. Since carbon is found in abundance on the earth, it is good for mass production. Since carbon atoms form strong chemical bonds with silicon atoms, carbon has sufficient adhesion as a supporting substrate of the silicon film.
A p-type silicon layer 231 is deposited on the substrate 230 by CVD or the like. The silicon layer formed by CVD becomes polycrystalline silicon when the temperature exceeds 600.degree. C., but the grain diameters of the polycrystalline silicon thus obtained are not larger than one micron. In order to obtain improved performance of a solar cell, the polycrystalline silicon is melted and then recrystallized on the substrate 230 to increase the grain diameters. In this way, a silicon layer 231 in which the diameters of the crystal grains are twice as large as their thicknesses is obtained.
An n-type emitter layer is formed in the surface region of the silicon layer 231 by impurity diffusion or other means to form a p-n junction. Thereafter, a transparent conductive film, an anti-reflection film, or the like (not shown) is formed as occasion demands. Finally, a metal electrode 233 is formed to complete the solar cell.
In the solar cell shown in FIG. 51, since the silicon layer 231 is directly formed on the substrate 230 mainly comprising carbon, a small amount of impurities included in the substrate 230, such as calcium, iron, aluminum, or sulfur, is diffused into the silicon layer 231 while forming the silicon layer 231 or the emitter layer 232 at a high temperature. These impurities degrade the characteristics of the silicon layer 231 as a semiconductor, resulting in poor performance of the solar cell.
In order to solve this problem, the structure shown in FIG. 52 has been proposed. In this structure, a silicon oxide film 234 is inserted between the substrate 230 and the silicon layer 231. When the entire surface of the substrate is covered with the silicon oxide film 234, the silicon layer 231 and the substrate 230 are electrically isolated from each other. Apertures 235 are formed through prescribed portions of the silicon oxide film 234 to electrically connect the substrate 230 with the silicon layer 231.
In this case, although the silicon oxide film 234 serves as a barrier, blocking the impurities from the substrate 230, the impurities are diffused into the silicon layer 231 through the apertures 235, as in the structure shown in FIG. 51. Further, since adhesion between carbon and the silicon oxide film 234 is weak because of the silicon oxide film 234 disposed on the substrate 230, the silicon layer 231 easily exfoliates from the substrate 230 and the structure is easily destroyed, resulting in low reliability and poor production yield.
The reason why the adhesion between carbon and the silicon oxide film 234 is weak will now be described. Carbon reacts with the silicon oxide film at a high temperature and evaporates as carbon dioxide. While forming the silicon layer 231 or the emitter layer 232 at the high temperature, carbon reacts as described above at the boundary between the substrate 230 and the silicon oxide film 234 whereby the chemical bond formed between the substrate 230 and the silicon oxide film 234 is weakened. As a result, only weak adhesion, depending on physical bonding between them, is obtained. Since a chemical bond is formed between the silicon and silicon oxide, adhesion between them is far stronger than the adhesion between the carbon substrate and the silicon oxide film.
When the silicon oxide film 234 is present on the rear surface of the silicon layer 231, light, which is incident on the solar cell and travels through the silicon layer 231 without being absorbed, is reflected and returns into the silicon layer 231 due to a difference of refractive indices between the silicon layer 231 and the silicon oxide film 234. The effect of reflection at the rear surface of the silicon layer 231 is larger when the silicon oxide film 234 is present than, as in the case of FIG. 51, where the silicon layer 231 is directly in contact with the substrate 230.
As described above, the reflection at the boundary between the silicon layer 231 and the silicon oxide film 234 is improved by inserting the silicon oxide film 234. However, the substrate 230 comprising mainly carbon is present beneath the silicon oxide film 234 and absorbs light well because it is black. Therefore, the light passing through the silicon oxide film 234 is not reflected at the boundary between the silicon oxide film 234 and the substrate 230 and the reflection at the rear surface of the silicon layer 231 is not much improved.
In order to complete the structure of the solar cell shown in FIG. 52, the apertures 235 penetrating through the silicon oxide film 234 are required. In conventional patterning of a silicon oxide film, the silicon oxide film is covered with a resist film having a prescribed pattern and etched by dry or wet etching. Since a resist film is used, a wet process is necessary even when using dry etching.
However, the substrate 230, comprising mainly carbon, is porous and absorbs water. Therefore, once the substrate 230 has passed through a wet process, it is difficult to remove water absorbed into the substrate 230. Such a substrate adversely affects laser processes and reduces the reliability of the solar cell.
FIGS. 56 to 58 are cross-sectional views showing conventional solar cells having concavo-convex structures. FIG. 56 shows a monocrystalline solar cell, FIG. 57 shows an amorphous silicon solar cell, and FIG. 58 shows a thin film solar cell.
In the monocrystalline solar cell shown in FIG. 56, a concavo-convex configuration is formed by anisotropic etching using potassium hydroxide (KOH) or the like on the surface of a silicon (100)-oriented monocrystalline wafer 240. Impurities are diffused into the wafer from the surface to form a p-n junction 241. Then, a grid electrode 242 is formed. In order to improve the efficiency of the solar cell, an anti-reflection film (not shown) may be formed on the wafer having the concavo-convex configuration or a back surface field (not shown) may be formed on the rear surface of the wafer by diffusing impurities. In this solar cell, light 243 incident on the concavo-convex part of the wafer is converted into electricity in the monocrystalline layer 240 and then output from the grid electrode 242 and a rear electrode (not shown) on the rear surface of the monocrystalline layer 240. In this structure, light reflected at the wafer surface also enters into the wafer, as shown by arrow 244, due to the concavo-convex configuration of the wafer surface so that less of the incident light is reflected away from the surface, thereby improving light-to-electricity conversion efficiency.
In the amorphous silicon solar cell of FIG. 57, a transparent electrode 246 is formed on a glass substrate 245. The transparent electrode 246 is formed by sputtering, vapor deposition, chemical vapor deposition, or plating SnO.sub.2, ZnO, ITO (In.sub.2 O.sub.3 :SnO.sub.2), or the like and the concavo-convex configuration is formed by selecting the formation conditions. Then an amorphous silicon film 247 is formed on the transparent electrode 246 by plasma CVD or the like. Further, a rear electrode 248 comprising, for example, Al or Ti/Ag is formed on the amorphous silicon film 247. In the solar cell thus formed, light is incident on the glass substrate 245 and electricity is output from the transparent electrode 246 and the rear electrode 248. In this structure, the angle of the incident light is varied by the concavo-convex part of the transparent electrode 246 so that the light travels a longer distance in the active layer 247 and a larger quantity of the light is absorbed, thereby increasing the light-to-electricity conversion efficiency of the solar cell.
In the thin film solar cell of FIG. 58, a heat-resistant substrate 250 comprising alumina, a conductive ceramic, or the like is mechanically shaped to form a concavo-convex configuration. Then, a polycrystalline silicon thin film 251 is formed on the substrate 150 and impurities are diffused into the polycrystalline silicon thin film 251 to form a p-n junction 252. Thereafter, a grid electrode 253 is formed. In the thin film solar cell thus formed, since the wafer surface has the concavo-convex configuration like the solar cell shown in FIG. 56, less incident light is reflected at the surface, i.e., a larger quantity of light is absorbed. Further, when the light is reflected at the boundary between the active layer and the substrate 250, the angle of the reflection varies so that the light travels a longer distance in the active layer, increasing the light-to-electricity conversion efficiency of the solar cell.
In the solar cell using the crystalline silicon wafer shown in FIG. 56, the concavo-convex configuration is easily formed by chemical treatment. In the amorphous solar cell shown in FIG. 57, the concavo-convex configuration is formed only by changing the conditions for forming the transparent electrode. However, these methods cannot be applied to the thin film solar cell shown in FIG. 58 in which the active layer is formed by heating polycrystalline silicon or the like to a high temperature. In FIG. 58, the concavo-convex configuration is formed by mechanically cutting the heat-resistant substrate or by processing the substrate with a laser or the like, resulting in a complicated and costly production process.
FIGS. 59(a) to 59(f) are perspective views showing process steps for producing the conventional thin film solar cell disclosed in, for example, Solar Cells, 29 (1990), pages 257-266. This thin film solar cell is finally separated from a substrate. In the figures, reference numeral 260 designates a p-type monocrystalline silicon substrate. V-shaped stripe grooves 261 are formed on the surface of the substrate 260. An n-type crystal silicon layer 262 is disposed on the substrate 260. V-shaped stripe grooves 263 are formed on the surface of the n-type crystal silicon layer 262. A glass substrate 264 is bonded to the crystal silicon layer 262.
A p-type polycrystalline silicon substrate 260 shown in FIG. 59(a) is first prepared. Then, as shown in FIG. 59(b), a plurality of V-shaped stripe grooves 261 are formed in the substrate 260 by anisotropic etching. As shown in FIG. 59(c), a low impurity concentration n-type silicon layer 262 is epitaxially grown on the substrate 260 having the stripe grooves 261. The surface of the silicon layer 262 is flattened by polishing when necessary. Then, as shown in FIG. 59(d), a plurality of V-shaped stripe grooves 263 perpendicular to the stripe grooves 261 are formed on the surface of the silicon layer 262 by anisotropic etching. Thereafter, a p-n junction is formed in the surface region of the silicon layer 262 and a protection film and an electrode (not shown) are formed on the silicon layer 262. Then, as shown in FIG. 59(e), a glass substrate 264 is bonded to the silicon layer 262. The glass substrate and an adhesive agent for the bonding should have properties so as not be etched away in the next etching step. Then, the monocrystalline silicon substrate 260 is removed by selective etching, as shown in FIG. 59(f). A protection film and an electrode (not shown) are formed on the rear surface of the silicon layer 262 to complete a solar cell. This method is not suitable for mass production because the solar cells are produced one-by-one.
FIG. 60(a) is a cross-sectional view showing a conventional solar cell module disclosed in Japanese Published Utility Model Application 60-194355. In FIG. 60(a), a solar cell base 270 comprises monocrystalline or polycrystalline silicon. The solar cell includes an upper metal electrode 271a for collecting current generated by light and a lower metal electrode 271b serving as a counter electrode. A lead wire 272 is connected to the upper electrode 271a of a solar cell and to the lower electrode 271b of another solar cell by solder 273 or the like. A plurality of solar cells thus connected are sealed by a resin 274 between an upper glass sheet 275 and a rear plate 276.
FIG. 60(b) is a cross-sectional view showing a conventional thin film solar cell module disposed on a conductive substrate 280. A power-generating layer 281 comprising a semiconductor thin film is disposed on the substrate 280. An upper metal electrode 282a is disposed on the power-generating layer 281. A lower electrode 282b is disposed on the rear surface of the substrate 280. One end of a lead wire 283 is connected to the upper electrode 282a of a solar cell while the other end is connected to the lower electrode 282b of another solar cell by solder 284 or the like whereby the two solar cells are electrically connected. A plurality of solar cells thus connected are sealed by a resin 285 between an upper glass sheet 286 and a rear plate 287.
When light enters into the solar cell 270, electrons and holes are generated in the solar cell. The electrons and holes are transported to the upper side (surface side) and the lower side (rear surface side), respectively, by an internal electrical field in the solar cell and then collected by the upper electrode 271a and the lower electrode 271b, respectively, and are output as an electrical current. When the solar cell 270 comprises silicon, the operating voltage for generating the maximum output is about 0.5 V per cell. An ordinary power module has several to several tens of solar cells connected in series. In order to realize the series connection of solar cells, the lead wire 272, one end of which is fixed to the upper metal electrode 271a by the solder 273, is drawn beneath an adjacent cell and the other end of the lead wire 272 is fixed to a part of the lower metal electrode 271b of the adjacent cell by the solder 273. In this way, a plurality of solar cells connected in series are sealed by the resin 274, the surface glass 275, and the rear plate 276, resulting in a power solar cell module.
In the case of the thin film solar cell formed on the dielectric substrate 280 shown in FIG. 60(b), in order to connect a plurality of solar cells in series, the lower electrode 282b is formed on a part of the rear surface of the dielectric substrate 280 and then an end of the lead wire 283, the other end of which is fixed to the upper electrode 282a of an adjacent cell, is fixed to the lower electrode 282b by the solder 284.
FIGS. 61(a) and 61(b) are schematic diagrams showing the combined solar cell element disclosed in Japanese Published Patent Application 59-3980. As shown in FIG. 61(a), a surface electrode 291a of a solar cell 290 is connected to a rear electrode 291b of an adjacent solar cell by solder or a conductive adhesive agent 292. In this way, a plurality of solar cells 290 are stacked as shown in FIG. 61(a), realizing a series connection of solar cells. In this structure, although the series connection of solar cells is achieved in a simple process, the connecting portions are subjected to stress. Therefore, although this structure is applicable to power sources for domestic equipment, it is inapplicable to power modules using large cells, i.e., about 10 centimeters square, because of poor reliability.
FIG. 62 is a cross-sectional view showing the integrated amorphous solar cell disclosed in Japanese Published Patent Application 61-54681. In FIG. 62, transparent conductive films 301, amorphous silicon layers 302, and metal electrodes 303 are disposed on prescribed regions of an insulating substrate 300 whereby an upper electrode is connected to a lower electrode of an adjacent unit cell, realizing a monolithic series-connection on the same substrate. Lithography using a conventional photo-resist or laser patterning has been employed as a method for separating the unit cells from each other. These techniques require complex process-steps and production cost is high.
According to the background described above, a structure employing the connecting method as shown in FIG. 60(a) is generally used as a main current of a power solar cell module. However, the method for connecting solar cells in series shown in FIG. 60(a) requires the steps of:
1) soldering the lead wire 272, which is bent as shown in FIG. 60(a), onto the upper electrode 271a of each cell;
2) arranging each cell in a prescribed position so that an end of the lead wire of a cell may be in contact with a part of the rear electrode 271b of an adjacent cell;
3) turning all of the cells upside down; and
4) soldering the end of the lead wire onto the lower electrode 271b.
These steps are very complicated and cannot be easily automated.
In the solar cell module shown in FIG. 60(b), in addition to the above-described complex steps, it is necessary to form the lower electrode 282b on a part of the rear surface of the conductive electrode 280. In this case, the bonding strength between the conductive substrate 280 and the lower electrode 282b and the soldering strength between the lower electrode 282b and the lead wire 283 are not enough, resulting in an unreliable solar cell.
Japanese Published Patent Application 3-22574 discloses another method for connecting solar cells in series in which solar cells are connected by screws. This method also has a complex step of boring holes through a substrate for the screws. In addition, since the substrate supporting a power-generating layer comprises metal, it is not possible to form a power-generating layer with superior characteristics.